Amplification system



Patented Dec. 17, 1929 UNITED STATES COHEN', 0F BROOKLYN, NEW YLORK AMPLIFICATION .SYSTEM Application filed. September 8, 1924. ySerial No. 736,602,

My invention relates broadly to amplification of electric-al energy by electron tube circuits and more particularly to a system of amplification Where a plurality of tubes in 5 an ainplilier may be operated without interference due to the setting up of undesired oscillatory currents within the amplier eircuits.

Oene oi the objects of my invention is to 3 3 provide an amplification system of high stability operating to increase the amplitude of incoming signaling energy but not being inluenced by local oscillation or regeneration within the amplifier circuits.

Another object oi my invention is to provide means for connection in various types oi electron tube `amplificati'on circuits" for stabilizing the circuits, enabling the elicient operation ci' the system at the signaling frego quency without the generation of oscillations within the circuits by reason of the Vinherent capacity coupling between tube electrodes which normally causes i'ee'd back and regeneration, the interfering currents being transgv; ieri-.ed from one electron tube circuit to another by reason of lie resonance which exists between the several stages of the apparatus.

Still another object oi' my invention is to provide a shunt circuit arrangei'nent related to each tube for preventing the spilling over of the tube into a condition oi regeneration by reason of self-excitation, thereby secur ing true repeater action throughout a plurality of tubes.

A further object of my invention topvovide a circuit arrangement including resistance and capacity in shunt with the tube electrodes 4functioning to shift the phase oi interfering currents in the amplifying system to va value so different from the Jhaseot the received signaling currents that ihe tendency of the tube to oscillate or regenerate is an.- nul'led.

My invention further resides in the provision of a phase displacements circuit for undesired oscillatory currents for connection in a variety oi different electron tube ampli tying systems, whereby the characteristics of the interfering current-.S are so alt-cred with respect to the Characteristics of the signal-A ing currents that the tubes will ,ceae oscilf lating or regenerating and operate as true reT peaters.

My invention will be more clearly under--- stood from the following specification by ref- )liiier circuit o ieratino" in con`unction with.

an .electron tube detector at a radio receiver.

Heretofore in the art amplifying systems have embodied Certain difficulties causing un.- stable operation. This has been particularly true in circuits employing tuned radio frequency amplification where multi-stage am? plifiors are used. One of the main causes of trouble is the inherent capacity coupling bef tween tube electrodes, causing feed back and regeneration, a condition which exists when resonance between circuits occur at each stage in the system,

Several methods have been evolved for counteracting or neutralizing this dilllulty. One'method is the utilization of .a third elec? tro-motive force lor counteracting the disturbing F. M. F. This .disturbing M. Fiis usually derived from the grid and plate cir cuits of the amplifier by means of an electrof magnetic coupling between the grid and plate circuits. An E. M. F. is thus derived, whose magnitude, and phase is equal opposite to the disturbing M, F. The ditlculty of this method is the necessity of varying or changing the coupling Jfor Idillerent operating frequencies for the adjustment is very critif cal.V Another method heretofore proposed is to balance the electrostatic capacity .of the tube. In this method the usual Wlie-atstoe bridge arrangement is used wherein small balancing capacities are arranged in the circuit in such manner as to balance out the troublesome capacity coupling in the circuit.

its phase relation with respecttothe other existing electromotive forces comprising the signaling currents. The disturbances einst-4 iiig in radio frequency amplifier valves is due to the mutual capacity Vof theelectrodes;`

namely, the capacity between grid and lilament, filament'and plate and plate and grid. lf the circuits in which the tube electrodes are connected are caused to be in resonance with each other, the mutual capacity coupling between the grid and plate is sufficient to set the tube in an ,oscillating state,` This may also be explained due to the fact that the field ofthe grid-plate condenser reacts on the grid filament condenser so as to produce a voltage in this condenser in phase with the impressed E. M'. F. This produces negative conductance in the input circuit, thus inducting a transfer of power from the'plate to the grid circuit with no other coupling between the grid and plate other than the existence of the mutual capacity coupling between the grid and plate.

An analytical analysis of the phenomena that'takes place in an electron tube and particularly the part that vplays the important rle of causing the disturbances due to the,

mutual capacity may be considered from Fig. llof the drawings. In this diagram G, P, and

F correspond to the grid, plate and `filament' of an electron tube respectively. The capacity Cl corresponds to the electrostatic capacity between grid and filament electrodes; C2 the electrostatic capacity between the plate and filament electrodes and C3 the mutualy electrostatic capacity of grid to plate. The

impressed voltage@l may be derived fromthe secondary s of a coupling transformer having an inductance L1. The corresponding sym# bols for the remainder of the circuit are :l

rg=input grid resistance acg=alternating plate voltage where ais the amplifying constant of the tube. i

`rvs-plate resistance i rf-load resistance `The voltage of the grid willvdepend upon thefinput resistance and this resistance is al function of the `load inductance, Aplate resiste ance, grid capacity andthe mutual capacity C3. The input effective resistance may beeX- pressed as i" 'Itcanv be shown that for pure inductance in load resistance is It can be shown that as the angle p decreases, the negative yalue of rg decreases and finally becomes positive. It is apparent 'therefore that if the phase angle is caused to approach a value where the input resistance is such as to prevent-the tube from oscillating and at the same time permit the tube to operate as a truerepeater, then and only then will the system in question perform the duties required.l I accomplish the displacement of the.` phase angle of the undesired currents with respect to the signaling currents by the arrangement shown in Figure 2 where resistance R4 and condenser C4 are shunted across the mutual capacity C3 between the Vgrid and' plate electrodes. The tendency of the tube tooscillate depends upon the mutual capacity C3 yand the phase relation of the currents in the input and output circuits. The phase angle of the currents lin the input and output circuits between'the plateand grid electrodes being altered by reason of the change of the effective coupling capacity G3, the tube will cease oscillating or regenerating.` The shunt resistance R4 and the 'condenser Cil to the coupling capacity C3 may be repre:

sented by the following equation:

gai tain.-

0 v where p=phase angle hoztotal susceptance i go=conductance The proper constants can be computed forvv the determination of the shunt resistance and the capacitance for the proper phase angle in o rder thatthetube madeffunction as a true radio frequency repeater withoutv falling into f uw condition of osciiuuon. The function of the `resistance is thereforeH to change the phase angle of the coupling current with res spect to its voltage, to causethe absorption of the feed back power of the plate circuit to the grid circuit. as rapidly as itis generated and to enable the grid to be negatively biasedby energy derived from the plate circuit. The

. application ofthis princ'ple to electron tube A repeating ampliiiers istherefore apparent f since the drawback with lradio frequency am.-

` pliiiers, particularly for frequencies greater than 10, is the tendency to regeneration and mariees sustained oscillation', a condition of instability. This condition arises when resonance of the circuits is approached tothe incoming carrier frequency. f

rl'he theoretical principles illustrated in Figs. 1 and 2 are shown in Fig. 3 as applied to a tuned radio frequency amplification sys-i tem wherein reference character 1 designates a receiving antenna system with ground connect-ion 2 including in 'circuit therewith the primary winding 3 of coupling transformer 4. The secondary winding 5 of transformer' lis connected to the input circuit of the first of radio frequency amplification including electron tube 6. Electron tube 6 is shown containing filament electrode 7, grid electrode 8 and plate electrode 9. The input circuit 'of tube 6 includes secondary winding 5 tuned bycondenser 10. The output circuit of tube 6 includes primary inductance 11 of coupling transformer 16 connected to bus 12 and thence to the source of B battery potential represented at 15. Filament electrode 7 is energized from battery 15. Secondary winding 17 of coupling transformer 16 is tuned by condenser 1 8 and connects to the input circuit or across the grid and filament electrodes of electron tube 19 forming the second stage of radio frequency amplification 'in the amplifying system. The output circuit connected across the plate and filament electrodes contains primary winding 20 of coupling transformer 21 and source of B battery potential 141. The secondary winding 22 of the coupling transformer 21 is tuned by variable condenser 23 and connects to the input circuit or across the grid and filament electrodes of tube 24 forming the third stage of radio frequency amplification in the amplifying system. The output circuit of tube 211 across the plate and filament electrodes thereof includes primary winding 25 of coupling transformer 26 connected through bus 12 with B battery la. rlhe secondary winding 27 of coupling transformer 26 is tuned by variable condenser 28 and connects to the input circuit of the detector tube 29 including grid condenser and grid leak 30 in the grid circuit thereof and the responsive device 31 in the output circuit thereof connected to a tap 32 on B battery 14. A by-pass condenser 33 of relatively high capacity is connected across the filament and plate battery 14 and 15 of the amplifying system. The plat-e and grid electrodes of each of the tubes is shunted by the phase displacement circuit Ri and C.,V as previously illustrated in Fig. 2. This circuit is so related to the input and output circuits of the tubes as to bias the grid electrode negatively at the same time that the circuit operates in conjunction with the inherent capacity existing between the grid and plate electrodes for shifting the phase of the interfering currents with respect to the phase of the signaling currents thereby destroying any tendency of the disturbing currents t inter'- fere with the signaling currents.

As heretofore stated, byproperly adjusting this shunt circuit, the tube of each stage is prevented from spilling over and regenerating at the same time that a negative'potental is placed on the grid. From actual practice as obtained from tests, the value ofthe capacity C.L should be approximately 200 times as great as the grid plate capacity of the tube. The resistance R.L was found to vary from 25,000 'to' 50,000 ohms. The proper value of resistance has to be determined by trial depending upon the tube characteristics and the tube impedance. However, once found itbecomes fixed. The entire circuit remains stable in operation independent of the 'frequency and setting of each respective amplici'rcuit. On account of "the true repeater action obtainable in this system, without the usual oscillations and regeneration obtained with other systems of radio frequency amplification, it becomes possible to utilize step-'up voltage transformation'in the coupling transformers Z1, 16, 21 'and 26. Transformation ratio of 6:1 between primary and secondary winding have been used with success. The electromagnetic coupling between primary and secondary windings must be rigid, that is a tight and close coupling must be employed. The distributed capacity of the primary ywinding must be exceeding small and its resistance must vbe kept downto a minimum. It is important that theelectromagnetic and electrostatic coupling between each transformer be kept at "a minimum. To jac'- complislrthis it was found necessary to nimmt each transforme'rso that their planes are 'at right angles to eac-h other. At times it was found necessary t'o shield each transfori'ne'r in order to reduce'the electromagnetic stray field of each transformer and it was vfound that the 'physical dimensions of the windings had to be kept small, all of these factors operating to decrease the effects of capacity coupling between the tube electrodes.

'It is apparent that in such a system asheiein described that tuningof each stageis necessary in order to obtain maximum sensitivity and minimum interference. However, the difficulty of adjusting a number of controls may be yovercome by tuning the entire systern byeither one or two controls by mechanically coupling 'all the timing condensers Vof each st'agcand adjusting the" circuit by means of a single knob. The antenna coupling transformer l sheuld be tunedindependently of the radio 'freq'uenc'y transformers 16, `21 and 26. The sensitivity and selectivity ofv the set is Vthus'preserved while the number of tuning contrle is reduced toa minimum.

lirbrder to further increaseithefselectivity and better th'e stability of the amplifying system linay 'employ' the circuit arrangement illustrated in Figa where the phase displacement circuits also include a portion of the inductance of the input and output circuits. I have represented the phase displacement circuit C4 and R4 in the first stage of radio frequencyamplification' in shunt with the input and output circuits of tube 6 including a portion -ot secondary inductance 5 and a portion 36 of primary inductance 1l. The' phase Vdisplacement circuit tor tube i9 has been represented as including capacities Ci and resistance Ri and portions 37 and 38 oi' secondary coupling coil 17 and primary coupling coil 2O respectively. As indicated at 39 the resistance R., may be made variable for controlling the phase of the shunt circuit.

The circuit arrangements shown are to be considered in the illustrative sense rather than in the limiting sense. I may apply the principle of my invention to a variety o'll electron tube circuits. The invention is particularly adaptable to reiieX circuits for in this typeof double duty circuit it is very ditiicult to prevent the tubes from 'falling into oscilv lation. I have successfully applied my invention to a reflex set including three stages of tuned radio frequency amplification, crystal detector, and two stages of audio irequency ampliiication with very stable operation without indication ot oscillations or regeneration. `lith the resistance R4 constructed so that as to be readily varied perfect control of the stability of amplifier circuits is obtainable with tubes having different characteristics and it'becomes possible to adjust each circuit toa highly sensitive condition whereby high amplication is obtained. desire that it be understood that the principle of my invention may be applied to various amplilication circuits, as for example, the intermediate amplifier circuit of a superheterodyne receiving system or to power ampliiiers in transmitting circuits and that I intend no limitations upon thev invention l other than are imposed 'by the scope ot the appended claims.

What I claim and desire 'to secure by-Letters Patent ot the United States is as iollows:

l. An amplification system comprising in combination a plurality of electron tubes, each having grid, ilainent and plate electrodes, input and output circuits interlinking the electrodes ot' one ot' said electron tubes with the electrodes ot a succeeding eleccombination a plurality of electron tubes, each having grid, lilament and plate electrodes, input and output circuits interlinking the electrodes of one of said electron tubes with the electrodes of a succeeding electron tube, inductances in'each of said circuits and means comprisingl a capacity and resistance operative independently ot' frequency c ianges in said tube circuits connected between points intermediate the ends of each of said inductances in the path of interfering currents and operating to displace the phase of interfering currents with respect to the phase of signaling currents in said ampliiication system.

3. An amplification system comprising in combination a plurality of electron tubes, each having grid, filament and plate electrodes having inherent capacity coupling therebetween, input and output` circuits interconnecting the electrodes ot one of said electron tubes with the electrodes of a succeeding electron tube, inductances in each of said circuits and av circuit electrically related to the inherent capacity coupling of said electrodes operative independently of frequency changes in said tube circuits and connected `*between points intermediate the ends of said inductances in the path of interiering currents for displacing the phase of interfering currents with respect to the Y SAMUEL COHEN. 

